1. Field of the Invention
The present invention relates to a novel low-current multi-state current-switching magnetic memory, and, more specifically, to a magnetic memory element employing the torque generated by the polarized spins of electrons to move a free layer domain wall relative to a magnetic tunnel junction.
2. Description of the Prior Art
Computers conventionally use rotating magnetic media for data storage in the form of magnetic hard disk drives (HDDs). Though widely used and commonly accepted, such hard disk drives suffer from a variety of deficiencies. Access latency, higher power dissipation, increased physical size and inability to withstand any physical shock justifies a new type of storage device. Other dominant semiconductor based storage devices are DRAM and SRAM which are volatile and very costly, but have faster random read/write time. Solid-state-nonvolatile-memory (SSNVM) devices like NOR/NAND flash memory provide higher access time, higher IOP speed, lower power dissipation, smaller physical size, and higher reliability, but at a cost which tends to be generally multiple times higher than HDDs.
Although NAND flash memory is more costly, it has replaced HDDs in many applications such as digital cameras, MP3 players, cell phones, and hand held multimedia devices. However, as process geometry is getting smaller, the design of NAND flash memory and DRAM memory is becoming more difficult to scale. For example, NAND flash memory has issues related to capacitive coupling, few electrons/bit, poor error-rate performance, and poor reliability due to low read-write endurance. It is believed that NAND flash memory, especially multi-bit designs will be extremely difficult to scale below 45 nm-lithography. Likewise, DRAM has issues related to scaling trench capacitors, necessitating complex designs that are becoming difficult to manufacture, and leading to higher costs.
Currently many platforms use combinations of EEPROM/NOR, NAND, HDDs, and DRAM as a part of the system design. Use of multiple memory technologies in a single product will add to the design complexity, time to market, and the final cost. For example, a handheld multi-media device which incorporates NAND Flash, DRAM, and EEPROM/NOR flash will have additional levels of design complexity, cost more, and take longer to reach the market than a device incorporating fewer unique memory technologies. Also, incorporating multiple memory technologies increases the device's footprint, an undesirable property for a handheld device.
There has also been extensive effort in development of alternative technologies such as Ovanic RAM (or phase-change memory), Ferro-electric RAM (FeRAM), Magnetic RAM (MRAM), nanochip, and others, to replace memories used in current designs. Although these various memory/storage technologies have created many challenges, there have been great advances made in this field in recent years. Magnetic-random-access-memory (MRAM) seems to lead as the best candidate to replace all types of memories as a universal memory solution. Recently, low capacity MRAM, which relies on a magnetic field to switch the memory elements, started shipping. Numerous scientific papers have shown that direct electrical current can also be used to switch the memory elements. There is a tradeoff between low-switching current, and reliability of the memory associated due to thermal stability.
The current-switching tradeoff arises due to the interaction between magnetic moments and the electrical transport current. At least two dominant mechanisms have been proposed, namely, (1) current induced magnetic field and, (2) spin-torque from the current spin polarization. While the current induced effect through the magnetic field is directly proportion to radius (r), the later spin-torque effect is proportional to r2, where r is the distance from the center of current carrying device. Incidentally, the torque generated by spins through the momentum transfer of tunneled polarized spins from the fixed layer, which opposes the “intrinsic” damping of spins of the free-layer. At sufficient current this can reverse the direction of the magnetization in the free layer. The critical current required for such switching:
                    Ic        =                                            Ic              0                        ⁡                          [                              1                -                                                      (                                                                                            k                          b                                                ⁢                        T                                                                                              K                          u                                                ⁢                        V                                                              )                                    ⁢                                      ln                    ⁡                                          (                                                                        t                          p                                                                          t                          0                                                                    )                                                                                  ]                                .                                    Equation        ⁢                                  ⁢        1            Where Ic0 is the critical switching current density without thermal fluctuation; kb is the Boltzman constant; T is the temperature; Ku is the effective uniaxial anisotropy; V is the volume of the free-layer; t0 is the inverse of the procession frequency of the spin (less than 1 ns); tp is the pulse width of the switching current.
Equation 1 shows that one way to reduce the critical switching current density is by reducing either Ku or V of the free-layer. Secondly, the switching current can be reduced by utilizing a thinner free-layer, but this may compromise reliability by making the memory cell thermally unstable. A memory element with a free-layer having a higher KuV is more thermally stable at higher temperatures. A general rule of thumb is that the magnetic energy, KuV, of the free-layer be greater than about 80 kbT where, kb is the Boltzmann constant and T is the ambient temperature.
What is needed is a novel memory that has high tunneling magneto-resistance (TMR) while having flexibility in the selection of resistance-area product (RA) and lateral dimensions of the magnetic tunnel junction. A high TMR is highly desirable as it enables high easier sensing between the two states, while flexibility in RA design with lateral size enables scalability, as well as improving reliability.